There's also the possibility that Mace Windu is still alive out there somewhere. He got electrocuted, lost his hands, and got thrown out of a building. If Luke could survive what happened to him on Cloud City after (at most) a few months training, there's no reason to think one of the most powerful Jedi Masters at the time of the purge couldn't have survived the injuries we saw Windu take.
Not that this is really my idea -- I've seen people talking about how he should have survived and shown up in the EU in the past, and it's a passable argument. Not compelling enough to say he /did/ survive, but a good enough excuse if he happens to show up at some point.